Lift pump control systems may be used for a variety of fuel system control purposes. These may include, for example, vapor management, injection pressure control, temperature control, and lubrication. In one example, a lift pump supplies fuel to a high pressure fuel pump that provides a high injection pressure for direct injectors in an internal combustion engine. The high pressure fuel pump may provide the high injection pressure by supplying high pressure fuel to a fuel rail to which the direct injectors are coupled. A fuel pressure sensor may be disposed in the fuel rail to enable measurement of the fuel rail pressure, on which various aspects of engine operation may be based, such as fuel injection.
However, the inventors herein have identified potential issues with such systems. Lift pump pressure sensors may degrade. In particular, they may fail in-range while reading a higher pressure than actually is present. As a result, the closed loop pressure control system may drop that pumped voltage in response to the pressure sensor output reading falsely high. The lowered lift pump voltage has a commensurate drop in lift pump pressure. In particular, the lift pump pressure may drop below the fuel vapor pressure. Since the lift pump pressure is the same as the inlet pressure of the downstream high pressure fuel pump, the drop in lift pump pressure below the fuel vapor pressure results in the high pressure fuel pump sucking in fuel vapor. The presence of fuel vapors at the pump inlet of the high pressure fuel pump can result in a precipitous drop in fuel rail pressure, causing the engine to stall.
In one example, the above issues may be addressed by a method comprising: adjusting fuel lift pump operation in response to a lift pump pressure sensor downstream of the lift pump and upstream of a high pressure pump; and operating the lift pump with a minimum lift pump voltage when a commanded lift pump voltage is below the minimum lift pump voltage. In this way, at least a minimum pressure may be maintained downstream of the lift pump under all pump operating conditions.
In one example, a fuel system includes a lift pump for delivering fuel from the fuel tank to a high pressure fuel pump. The high pressure fuel pump may be coupled to a fuel rail delivering fuel to cylinder direct fuel injectors. The lift pump may be operated predominantly in a continuous power mode. Therein, based on a fuel pressure and fuel flow rate required to meet the fueling demand, a voltage (or speed, current, duty cycle, torque, or power) applied to the lift pump may be determined. For example, as the commanded fuel pressure increases, the command and pump voltage may also be increased, and likewise, as the commanded fuel pressure decreases, the commanded pump voltage may also decrease. However, a minimum clip may be applied to the pump voltage to enforce a minimum lift pump pressure. The minimum pressure, and corresponding minimum pump voltage, may be determined based on fuel vapor pressure and fuel flow rate. In other words, if the commanded pump voltage is below the minimum pump voltage, a controller may override the commanded pump voltage and apply the minimum pump voltage instead. Since the lift pump pressure is controlled in a closed loop manner with a PID controller, during the clipping, the integral term may be transiently frozen or reset (e.g., to zero). The lift pump may additionally be operated in a pulsed mode wherein lift pump voltage is adjusted based on lift pump pressure estimated by a lift pump pressure sensor. However, by applying the minimum pump voltage during conditions when the commanded pump voltage is lower, the potential for fuel vapor generation at the inlet of the high pressure pump is reduced. This, in turn, reduces the need for frequent lift pump pulsing.
In this way, a low voltage clip is applied to a lift pump command to ensure that the fuel system always makes a minimum pressure. As such this ensures a basic function of the pump system. By enforcing a minimum voltage on the lift pump that is a function of the commanded lift pump pressure, the closed loop controller may account for pump degradation. In addition, fuel system operation is improved even during conditions when a lift pump pressure sensor output is unreliable. Overall, engine stalls due to ingestion of vapor pressure at a high pressure fuel pump inlet is reduced. Further, by reducing the need for frequent lift pump pulsing, fuel system energy consumption is reduced.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.